A conventional optical transmission system includes tributary stations and a plurality of repeater stations. The repeater stations are connected in series with predetermined intervals between two of the tributary stations in series. In such an optical transmission system, a signal is transmitted from one tributary station through the plurality of repeater stations to the other tributary station.
Each of the repeater stations includes an amplifier for amplifying a received signal, a photodiode for detecting an output light to be transmitted to the next stage to provide a sampling signal, and a control circuit for controlling the amplifier in accordance with the sampling signal. In the control circuit, the sampling signal is compared with a predetermined reference value to obtain the difference value thereof, and a control signal corresponding to the difference value is supplied to the amplifier. In accordance with the control signal, the amplification ratio of the amplifier is controlled. Thus, the received signal is amplified and supplied to the next stage.
In the optical transmission system of multi-repeating type, spontaneous emission noises which occur in the repeater stations are accumulated during the transmission, which cause S/N (signal to noise) ratio of the transmission signal to be lowered.
This results in the increase of bit error ratio. In order to meet a predetermined level of the S/N ratio providing a predetermined level of the bit error ratio, it is required that an output power of each repeater station is set high. On the other hand, if the output power is set high, non-linear optical effect of optical fiber can not be negligible.
For this reason, an output power of each repeater station is set in a range determined by the minimum value meeting an S/N ratio and the maximum value dependent on the non-linear optical effect of optical fibers.
According to the conventional optical transmission system, however, an output power of each repeater station is difficult to be set in the range determined by the above maximum and minimum values, because characteristics of optical fibers, optical direct amplifiers, optical signals, etc. are different even at initial state of installation, and in accordance with the deterioration due to the elapse of time.